The term “directional coupler” refers in general to a four-port passive microwave device, where a main line conductor (also called the “through” line) carries RF power. The main line conductor is in close proximity and is coupled to a secondary conductor by the electromagnetic field generated by the RF signal. The RF current flowing forward through the main line will induce RF current flow in the coupled conductor flowing in the opposite direction, and will only appear at one of the coupled ports (i.e., a signal current flowing from left to right on the main line will induce a signal current flowing from right to left in the coupled conductor and appear only from the left coupled output). As a result, the coupled output of forward and reverse flow of RF current will appear at different coupled outputs.
While it has been possible to construct TEM mode couplers operating over wide frequency ranges using stripline techniques on solid dielectrics (where the dielectric constant, also known as dielectric permeability, Er>>1), it has been most difficult to do so using thick conductors in air dielectric (Er=1). The inherent size of the transmission lines in air has limited usage of these components to narrow bandwidths. Known TEM mode components suffer from degradation due to non-TEM propagation, manifesting itself as resonance in the pass band of the coupler. Low power components can use microwave absorbers to suppress unwanted resonance, but at higher powers such absorbers cause passive intermodulation distortion, rendering them useless in many high power applications.
Known coupler structures include single section, multiple section and tapered designs, among others. A comprehensive summary of such structures is provided in M. A. R. Gunston, “Microwave Transmission Line Data”, Noble Publishing, 1997, ISBN 1-884932-57-6. Gunston describes coupled transmission lines with coupled conductors of circular as well as rectangular cross sections. J. A. G. Malherbe, “Microwave Transmission Line Couplers”, Artech House, 1988, ISBN 0-89006-300-1 describes couplers with tapered conductors.
Peter A. Razzi, “Microwave Engineering, Passive Circuits”, Prentice-Hall, 1988, ISBN 0-13-586702-9 (hereinafter “Razzi”) discusses the high-pass characteristics of tapered structures. The high pass performance of such couplers is explained using the equivalence principle that equates the reflection coefficient of the tapered transformer to the coupling of the corresponding tapered line coupler. This reflection has a high-pass characteristic. As discussed in Razzi, a method of changing impedance levels in a transmission system involves the use of a continuously tapered line in which the impedance of the coaxial line is gradually transformed from R1 to Zo by tapering. The input SWR remains low as long as the taper length is much greater than the operating wavelength. The higher the frequency, the better this condition is satisfied.
All of the above mentioned structures suffer from signal loss due to excess loss in the dielectric material that surrounds the conductors and to excess coupling to the enclosure walls.
U.S. Patent No. 4,139,827 (“High Directivity TEM Mode Strip Line Coupler and Method of Making the Same”) uses stripline technology and adds a matching post in between the decoupled ends of the coupled conductors to increase coupler directivity.
U.S. Patent No. 5,521,563 (“Microwave Hybrid Coupler”) uses microstrip technology and adds a cross-over design to the transmission lines which changes the output port.
U.S. Patent No. 5,063,365 (“Microwave Stripline Circuitry”) uses two tandem connected Stripline Couplers and adds a phase shift circuit between the couplers which changes the phase relation of the output signals from 90 degrees to 180 degrees.
U.S. Patent No. 3,883,828 (“High Power Coupler Synthesis”) describes the synthesis of a directional coupler using a stripline broadside-coupled transmission line in conjunction with uncoupled transmission lines (delay lines) to obtain an equivalent coupler circuit.
The above mentioned stripline and microstrip based structures suffer from signal loss due to the relatively small effective conductor cross-section areas in the coupling section and due to excess loss in the dielectric material that surrounds the conductors.